1. Field of Invention
The present invention relates to a surface sensing device for use in position determining apparatus such as for example, a coordinate measuring machine (CMM), a scanning machine or robot.
2. Description of Related Art
Coordinate measuring machines are used for measuring workpieces, and typically comprise an arm movable in three directions x, y and z relative to a table on which the workpiece is supported. Movements of the arm in each of the x, y and z directions are measured by transducers on the machine, so that the position of the arm relative to a datum position can be determined.
The surface sensing device may comprise a touch trigger probe, or an analogue or measuring probe, and in each case includes a stylus assembly having an elongate stylus with a workpiece-contacting tip.
The surface sensing device with which the invention is particularly concerned is an analogue device for mounting on an articulating head, and suitable for use in a high speed scanning operation.
An articulating head is known per se for example from European Patent No. 360853. The head is capable of rotating the axis of a stylus assembly mounted on it about two orthogonal axes. In use, the head is mounted on the arm of the machine with one of its axes aligned with the axis of the arm. Transducers associated with each of the rotatable axes of the head determine the orientation of the axis of the stylus assembly relative to the axis of the arm of the machine.
During a scanning operation the machine and/or the head cause the stylus tip to move over the surface of a workpiece, in accordance with instructions from the machine controller, to gather data about the profile of the workpiece surface.
From the signals provided by the measuring transducers of the machine and head, and from knowledge of the dimensions of the parts of the surface sensing device, a prediction can be made about the position of the centre of the stylus tip.
However, the stylus assembly is subject to bending due to contact with the workpiece surface and due to inertial forces while accelerating, and this bending makes the actual position of the centre of the stylus tip uncertain.
One way of reducing the uncertainty is to make the stylus assembly stiffer. However this adds weight to the stylus which is a disadvantage, particularly for a high speed scanning operation.
Also, in a scanning operation it is essential to provide some means of knowing that the stylus tip is on the surface. Prior scanning systems have detected the bending of the stylus assembly for this purpose, and also in an attempt to determine the deflection of the stylus tip caused by the bending.
Examples of stylus assemblies in which transducers are used to determine when a stylus has been deflected are described in the patent specifications set out below.
In European Patent No. 360853 strain gauges are attached to the stylus to measure the actual bending of the stylus from which an estimate of the stylus tip deflection can be made.
Similarly in U.S. Pat. No. 5,118,956 the strain in a fibre within the stylus is measured by an interferometric method, and stylus deflection can be estimated from this measurement.
Another example of detecting stylus deflection to determine that the stylus has contacted a workpiece is described in U.S. Pat. No. 5,103,572. This patent describes the use in a trigger probe, of a laser resonator, the external reflector for which is attached to the stylus tip. Deflection of the tip by contact with the workpiece causes a change in the resonator quality which can be detected.
None of the methods described in the above patents are capable of giving a direct measure of the lateral displacement of the stylus tip due to stylus bending. U.S. Pat. No. 4,574,199 discloses an optical contact sensor in which a light source in a stylus ball creates an image of an aperture on a detector array so that when the stylus ball contacts a surface the image of the aperture moves across the detector.
The requirement to put a light source in the stylus ball is impractical with small diameter styli and requires electrical power connections to be positioned in the stylus which is an inconvenience.
The present invention in one of its aspects provides a surface sensing device which includes a transducer system which enables the lateral displacement of the stylus tip to be measured directly at or near the centre of the stylus tip.
In a preferred embodiment of the invention the transducer system is an optical system including a light source which directs a light beam along the length of the stylus towards the stylus tip, and a position sensitive detector positioned to receive the beam and to produce a signal indicative of the amount of lateral displacement of the stylus tip due to bending of the stylus. The optical system preferably comprises a retro-reflective component positioned at or near the centre of the stylus tip, the position sensitive detector and the light source being positioned at the opposite end of the stylus.
In order to be suitable for use in a high speed scanning operation the stylus assembly of the surface sensing device according to the present invention is preferably lightweight and relatively stiff so that its natural frequency of vibration is high enough not to be excited by vibrations of the machine on which it is mounted. At the same time, as described above, it is important to have some flexibility in order to be able to detect that the stylus is in contact with the workpiece.
However, since the transducer system according to a first aspect of the invention is capable of measuring the actual displacement of the centre of the stylus tip, the apparently conflicting requirements that the stylus assembly should have relatively high stiffness and yet still have flexibility, can be resolved in accordance with further novel aspects of the present invention with some advantageous results.
In another novel aspect of the invention the stylus assembly of the surface sensing device comprises a relatively stiff stylus carrier to which a relatively flexible stylus is attached. The combination is such as to provide overall the required high natural frequency of vibration, while the relatively flexible stylus, is made as small and as light as possible.
Thus in a preferred embodiment the stylus carrier has a conical or curved trumpet shape which can be designed to provide high stiffness to prevent bending during a scanning operation. The flexibility and low mass of the stylus are achieved by keeping its diameter as small as is practically possible.
Since the uncertainty which might have been introduced by allowing greater stylus deflections has been substantially eliminated by measuring the lateral displacement of the stylus tip directly, the flexibility of the stylus itself can be maximised to reduce the scanning torque required to be produced by the articulating head.
Also because the sensing system in the stylus produces signals indicative of the amount of lateral displacement of the stylus tip, in accordance with another aspect of the invention, these signals can be passed to a servo system in the controller to servo the articulating head. The controller can thus ensure that the stylus tip remains on the surface of the workpiece, and that the bending of the stylus remains within acceptable limits to minimise the chance of damage to the stylus. The higher natural frequency of the stylus assembly also allows servoing of the head at a higher rate.